1. Field of the Invention
This invention relates to a magnetic disc cartridge which has a thin disc-shaped magnetic recording medium. Namely, a magnetic disc sheet is enclosed in a case made up of upper and lower shells in such a manner that the magnetic disc sheet is rotatable in the case. More particularly, the invention relates to a magnetic disc cartridge having improved shells.
2. Background Art
A floppy disc is well known in the art which is manufactured by forming magnetic layers on both sides of a disc made of a flexible polyester sheet. The disc is used to record signals on the magnetic layers with the magnetic head while it is being rotated. The floppy disc is advantageous in that it can be readily handled and can be manufactured at low cost. Therefore, the floppy disc is extensively employed as a recording medium for computers.
On the other hand, a micro floppy disc has been put in practical use. The micro floppy disc has been provided by utilizing the advantages of the floppy disc. That is, it offers the advantages of magnetic recording in that the magnetic recording medium, unlike a silver halide film, can be used repeatedly. Also the micro floppy disc is miniaturized and is better than the floppy disc in that it can be more readily handled.
In this case, in order to improve reliability, a hard case is employed as the outside case, and a slidable dustproof shutter is provided for the window of the case which receives the magnetic head. On the other hand, a still camera has been proposed in the art which uses a small floppy disc as its video recording medium. The magnetic disc sheet used by the still camera is small in diameter and has a hub at the center. The magnetic disc sheet is rotatably supported in the small case. Both sides of the case have openings at the center to expose the aforementioned hub and windows to receive the magnetic head. That is, the magnetic disc sheet is used in the form of a magnetic disc cartridge.
The case of the magnetic disc cartridge is formed by combining the upper and lower shells of plastic resin. The shells, being molded with extremely small wall thickness, are liable to be deformed, for instance, being curved inwardly of the case. If the upper and lower shells curve inwardly, the inner walls of the shells are rubbed by the hub, or the line of the resultant crease is strongly pushed against the magnetic disc sheet, thus making the running torque unstable during the rotation of the magnetic disc sheet. Furthermore, if the case is excessively bent, sometimes the magnetic disc sheet or the dustproof shutter is locked. Especially a dual-sided recording type magnetic disc cartridge suffers from a disadvantage that, when the case is inwardly curved, the magnetic disc sheet cannot satisfactorily contact the magnetic head of the magnetic disc recording and reproducing device.
Heretofore, in order to prevent the abovedescribed deformation of the case, the inside of the metal mold has been subjected to so-called "spherical correction" so that the two shells are curved outwardly of the case when molded. However, the spherical correction must be performed with considerably high accuracy, thus greatly increasing the manufacturing cost of the metal mold.